My Volvo S40 1998 is misfiring when cold and although it gets better with the longer I drive it its not right - after about 30 mins of driving it seems to be ok although there is some shudder and hesitation if I drop down gears and want extra power without being smooth with the gear change! The revs seem to be very high when I turn the engine on at about 1800RPM though that goes down to normal within a few minutes.

I took the car back in September to Volvo who could not find anything wrong but said it was most likely the fuel was too rich and the engine management unit needed replacing!

But as far as I am aware it never once misfired while they had it! a few days later I took it to another garage and again they could not find anything wrong at all!

Yesterday it started doing it again - and I have booked it into a local garage for monday but I am wondering what it could be that would cause it happen then vanish then come back again! it seems highly eratic!

Any thoughts? I can only assume that when thge garages had the car they checked all the connections, plugs etc and cleaned everything which may have helped? I don't know - I am not very technically knowledgable about cars so please keep it simple if you have an idea for me! Thanks.